The invention relates to a method of diffusion coating an iron, nickel, cobalt, or copper base alloy with an aluminum and silicon containing coating diffused onto the surface of alloys using a pack cementation process and the insert used in that process.
Pack cementation is a well known technique for applying diffusion coatings to metal surfaces. This process involves placing a pack mixture into close contact with the surface being coated and subsequently heating the entire assembly to an elevated temperature for a specified period of time. During heating the coating material diffuses from the pack onto the surface of the metal by a combination of chemical reactions and gas phase mass transport. Pack cementation is commonly used to apply aluminum diffusion coatings as well as to apply chromium diffusion coatings. A common pack mixture used to create a chromium coating contains chromium, an inert filler such as alumina, and a halide activator. Similarly a common pack mixture used to produce an aluminum coating consists of an aluminum source, a halide salt activator and an inert diluent or filler such as alumina. Davis in U.S. Pat. No. 4,904,501 teaches that ammonium chloride, sodium chloride and ammonium bromide can be used as activators.
Aluminum-silicon diffusion coatings are preferred over aluminum diffusion coatings for some applications because silicon in the coating improves hot corrosion and ash corrosion resistance and reduces brittleness of the coating. The art has developed several methods of applying an aluminum-silicon coating to ferrous metal articles. Most commercial processes that are used to apply aluminum-silicon diffusion coatings require separate diffusion steps for each element or use expensive masteralloys. Masteralloys of aluminum and silicon cost 3 to 4 times more than pure aluminum and twice as much as pure silicon on a weight basis. Consequently, those skilled in the art have been searching for a less expensive process, particularly one in which an aluminum-silicon diffusion coating is applied in a single step. Preferably, the process should not require any materials that are expensive or difficult to obtain. The process should be suitable for use on existing equipment and for large scale processing operations. Both U.S. Pat. Nos. 4,500,364 and 4,310,574 discloses processes in which a slurry coating is applied to the article followed by high temperature firing. Slurries are more difficult to handle than the more common powder mixtures used in most pack cementation processes. Japanese Patent application 54090030 discloses a process in which steel plate is buried in an agent comprised of aluminum powder, silica (SiO2) powder and a halide and then heated at 1000xc2x0 C. in a nonoxidizing atmosphere to apply an aluminum and silicon diffusion coating. Because of the low reactivity of the silica powder, the resulting coating would contain very little silicon. Therefore, the benefits of having silicon in an aluminum diffusion coating are not obtained.
We provide a method of diffusion coating iron-, nickel-, cobalt- and copper-based alloys by simultaneous deposition of aluminum and silicon coating using a pack mix containing pure aluminum, pure silicon and an ammonium halide activator. The components to be coated are placed in a carbon steel or high temperature alloy retort and the surfaces to be coated are covered by the pack mix. The retort may be heated to between 300xc2x0 to 400xc2x0 F. for one hour to remove any oxygen or moisture present. Then the retort is heated to an interior temperature of 1200xc2x0 to 2100xc2x0 F. and held at that temperature for a selected time period. That time period will depend upon the base alloy being coated and the required depth of the diffusion coating. After the selected heating period has passed the retort is rapidly cooled and opened. Then the aluminum-silicon diffusion coated parts are removed. The coated parts are then cleaned and, if desired, also grit blasted.